A tap used after it becomes dull works under great strain and has a tendency to chip or break, cut oversize or produce rough and poor quality of threads. The remedy is to regrind.
Regrinding of taps should, if possible, take place in a tap grinder and not be done by hand. Machine ground taps are more free cutting, make more accurate threads and keep the sharpness of the edge longer.
When tapping, all of the cutting takes place on the chamfer length. Generally it is sufficient to regrind just this part of the tap in order to resharpen it. The amount of relief on the chamfer is dependent upon the length of the chamfer. The table below gives a value of the size of relieving for tap No. 2 and 3.
|
|  |
The chamfer and accompanying relief on the top of the thread should be identical on all of the lands of the taps. If the chamfer is uneven, the result will be holes much larger than actual tap size, torn and misshapen threads, uneven wear and eventual breakage of the taps.
When the edges of the thread begin to dull or become nicked, the flutes should be reground. A flute grinder providing accurate indexing of the cutting edges should be used. Unless the indexing is accurate, the tap is apt to break or cut oversize.
Regrinding of the flutes can also be used in cases where there is a lack of suitable equipment for grinding of the chamfer.
|
Note when regrinding:-
- The correct rake angle must be maintained - see table below
- The equal division of the cutting edges must be maintained
- The diameter of the tap will be reduced.
- The lands will be reduced and therefore will be weaker
- Avoid forming burrs on the flanks of the thread form.
|  |
Never regrind a damaged tap or a tap with build-up on the thread flanks. |